1. Field of the Invention
The object of the present invention is an oil well production string hanger.
2. Description of Related Art
An oil well comprises a production string or tubing through which the hydrocarbons flow from the reservoir rock as far as the surface, and a casing which supports the wall of the well. Between the two, there is an annular space used for the control of the well.
The tubing hanger is fixed at the top of the well, at ground level or on a platform if it is a well open to the air or alternatively at the bottom of the water if it is a sub-sea well. Its functions are to anchor the tubing at ground level, to make a seal with the well head situated above, and to allow the tubing and the annular space to be plugged when the well head is withdrawn, the well head being equipped with all the valves necessary for the safety and the opening and closing of the well.
The hanger generally has two passages allowing communication with the tubing and with the annular space respectively. It also comprises, especially for sub-sea wells, passages for hydraulic fluids and for electric cables for controlling the well bottom equipment (valves, sensors).
The hanger is housed beneath the production well head (or alternatively in the drilling well head used during the drilling phase which precedes the production phase).
When the production well head is to be replaced (or the drilling well head is to be withdrawn at the end of drilling), it is essential, for safety reasons, first of all to seal off the tubing and the annular space using plugs and thus regain the seal which was provided before by the safety valves in the well head.
There are, at the present time, two types of hanger: the directional type which is orientated with respect to the axis of the well, and the type which is concentric with this same axis. Each has two passages allowing communication with the tubing and with the annular space respectively. They differ essentially in the method used to seal the annular space.
For the directional type, the seal is provided by a plug lowered down on a cable. This is a reliable system which allows the plug both to be fitted and to be removed, but it requires the use of a double sub-sea intervention string, in the case of a sub-sea well: one for the annular space and one for the tubing, and this increases the duration of the operation and the cost of the equipment (hanger, well head and strings are more complicated and therefore more expensive).
For the concentric type of hanger, the device for sealing the annular space is installed at the same time as the hanger, and then activated by an automatic system as the production head is withdrawn. This is a simple and quick system which requires just one sub-sea intervention string, but requires the well head to be removed and the entire tubing to be reassembled in order to replace the sealing plug which cannot be recovered using a cable. The concentric hanger is not widely employed on account of these drawbacks.